Mrs McDonald referred to Mr Varadkar's penchant for novelty socks and his reference to the film Love Actually during his first visit to Downing Street last year and said: "Leo is kind of smarmy."

Her jibe at the Taoiseach comes less than a day after she replaced Gerry Adams as Sinn Fein president during a special party conference in Dublin on Saturday. It is not the first time Mrs McDonald and Mr Varadkar have clashed.

In September the Taoiseach accused Mrs McDonald of being "very cranky" and compared her to French far-right leader Marine Le Pen during ill-tempered exchanges in the Dail.

When asked during an interview with Sky News on Sunday what word she would use to describe Mr Varadkar, Mrs McDonald responded: "Smarmy."

She added: "Leo is kind of smarmy. You'll have seen him in Number 10 talking about Love Actually and donning various pairs of socks. So, smarmy." Mrs McDonald said she was hopeful she would see a united Ireland within her lifetime as Sinn Fein president.

"I would be hopeful that in the course of my tenure as leader that, yes, we would secure a referendum on unity and we would win it. It's doable," she said.

She described Brexit as an "absolute disaster" and said it was "mutually incompatible" with the Good Friday Agreement.

"I sense a real resentment amongst Irish people that Ireland becomes collateral damage in a power play with the Tories in Dublin," she said.

Mrs McDonald said the bread-and-butter and political interests of Ireland "demand the entire island stay within the customs union and single market".

"It is alarming to hear the mantra from London 'we are gone, we are out' with no sense of the consequence of that."

With crisis talks due to resume at Stormont today aimed at restoring the power-sharing executive, Mrs McDonald said she was hopeful an agreement could be reached with the DUP.